The present invention relates generally to a brake control apparatus for use in motor vehicles, and more particularly to a brake control apparatus suitably employed for a diagonal braking hydraulic pressure supply system.
One known braking hydraulic pressure supply systems for braking wheels of a motor vehicle is a diagonal braking hydraulic pressure supply system in which a braking hydraulic pressure generated in the master cylinder and applied through a first hydraulic pressure supply system to the front-left wheel and the rear-right wheel and further applied through a second hydraulic pressure supply system, having the same structure as the first hydraulic pressure supply system, to the front-right wheel and the rear-left wheel. That is, for example, as shown in FIG. 1, a hydraulic pressure generated in a master cylinder 2 in response to depression of a braking pedal of the motor vehicle is supplied through a hydraulic pressure system 100 to a wheel braking cylinder 3a for the front-right wheel FR and a wheel braking cylinder 3d for the rear-left wheel RL and further supplied through another hydraulic pressure system 200 to a wheel braking cylinder 3b for the front-left wheel FL and a wheel braking cylinder 3c for the rear-right wheel RR. Each of the hydraulic pressure systems includes a hydraulic pressure control device 1a or 1b for allowing execution of two-channel anti-skid control, for example, and a proportioning valve 4a or 4b which is provided in a rear-wheel side hydraulic pressure passage.
One important problem in such a braking arrangement relates to the deterioration of travelling stability and increase in braking distance. That is, the proportioning valves 4a and 4b used in the prior art braking arrangement are of the characteristic-fixing type and the characteristics are determined such that the hydraulic pressure distribution between the front and rear side wheels is appropriate when running on a general dried asphalt road surface. For example, in the case of a front-wheel driven (FF) vehicle in which the front wheel has a spike tire and the rear wheel has a normal tire, the pressure in the front-wheel braking cylinder may be required to be more increased in order to shorten the braking distance of the motor vehicle. That is, in FIG. 2, irrespective of having the pressure distribution characteristic indicated by the solid line, the braking hydraulic pressures for the front and rear wheels are determined as a point B. According to conventional proportioning valves, when the braking hydraulic pressure indicated by the point B is applied to the front wheel, the braking hydraulic pressure indicated by a point C is applied to the rear wheel, resulting in a tendency to lock thereby deteriorating the travelling stability. On the other hand, when the braking hydraulic pressure indicated by the point B is applied to the rear wheel, the braking hydraulic pressure indicated by a point A is applied to the front wheel, resulting in a lack of the braking force and thus lengthening the braking distance.
A further problem arising in a two-channel anti-skid control apparatus in such a diagonal hydraulic pressure supply system is that there is no possibility that, if a different friction coefficient exists between the road surfaces for the left and right wheels, the hydraulic pressure in the wheel braking cylinder for the high-friction side wheel is controlled to be higher and the hydraulic pressure in the wheel braking cylinder for the low-friction side wheel is controlled to be lower. More specifically, assuming that the right side wheels are on the high-friction road surface and the left side wheels are on the low-friction road surface, it is preferred that the front-right wheel braking cylinder pressure is controlled to be relatively high and the rear-left wheel braking cylinder pressure is controlled to become relatively low. Furthermore, it is also preferred that the front-left wheel braking cylinder pressure is controlled to become relatively low and the rear-right wheel braking cylinder pressure is controlled to be relatively high (as indicated by a point D in FIG. 2). However, the former, due to the characteristic of the proportioning valve as described above, results in an excessive rear-wheel side braking force or an insufficient front-wheel side braking force, and the latter results in an insufficient braking force of the rear-right wheel when the braking pressure applied to the front-left wheel is set to an appropriate value and an excessive braking force of the front-left wheel, thereby causing the front-left wheel to become locked, when the rear-right wheel pressure is set to an appropriate value (as indicated by a point E in FIG. 2). Therefore, these problems cause difficulty to be encountered to meet the requirements of the appropriate execution of the anti-skid control and the traction control, i.e., wheel slip control, because of being difficult to make an appropriate hydraulic pressure distribution for the front and rear wheels.